OpticsFilter: The Rainbow Connection

August 14, 2024 4:44 AM

I got a new light fixture! It's a metal frame and the bulb is surrounded by hanging glass prisms. Help me figure out which way to hang the prisms for maximum rainbow effect?

They have your classic triangular prism shape, long and narrow, and drilled at one end for the little wire hanging loops. The holes are drilled such that I can hang them with either a flat side or an angle towards the bulb.

My question is: how can I get the maximum rainbow effect from this fixture? I know that I'd get the maximum *light* by hanging the prisms peaked side out, but would I get more rainbows if I hung them the other way?

(It's near a window so it will get some sun on it, but I'm interested in how to optimise it for when the bulb is turned on)

posted by Pallas Athena to Science & Nature (4 answers total)

Putting them peak facing in will give the longest length within the prism for the different wavelengths to become apparent.

However, look at the Prism (Optics) page at Wikipedia and the best examples of the prism really control all the extra light around the prism -- refraction spreads the light energy around. So you might point the prisms inward and then also add a mask inside the lampshade so that only thin strips of light emerge from slits you cut into card, aimed at the sides on the inward-pointing prisms like the classic image.
posted by k3ninho at 4:57 AM on August 14 [2 favorites]

also consider the space? unless this is going in the exact center of a room that is a perfect circle, there may be spots where rainbows intersect. slight adjustments may further maximize rainbow effects 🌈

sounds fun, enjoy!
posted by HearHere at 5:46 AM on August 14 [1 favorite]

Best answer: Rainbows are all about refraction, which is about how light bends when it moves between one medium (air) and another (prisms), and back out again. Rainbows are generated through a prism where the incident light (incoming) is at around 40 degrees to a flat face (on the trianglar cross-section). So if you have a true triangluar prism, pointing the peak into the bulb should mean that incident light is around 30-35 degrees to the flat face (assuming internal angles are 60degrees, and the light bulb is quite big versus the radius of the prisms - ie the diameter of the whole light fixture isn't greater than about 1 foot), which should get better rainbows that flat face in, which would have an incident light angle of around 90degrees, which science would say is way too high.

So, thinking science, peaks inwards is best! Although the interactions between all the prisms might end up changing things and potentially you can find cooler effects by mixing it up - maybe alternate is actually best!
posted by london explorer girl at 7:29 AM on August 14 [1 favorite]

You'll also get sharper brighter rainbows with a smaller light source! A clear bulb where the filament or LED is visible will work better than a frosted bulb, which smooths the light out into a big pleasant blob, but which would also blur the rainbows. Getting a bulb that's as close to a tiny dot of light as possible is similar to k3ninho's suggestion of using slits in cards to make the light source look skinny to the prisms.
posted by moonmilk at 7:36 AM on August 14 [1 favorite]